[llvm] [RISCV] Add invariants that registers always have definitions. NFC (PR #90587)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Does it necessary to use `getUniqueVRegDef` instead `getVRegDef` here?

It's a slightly stronger assertion to double check everything's in SSA, but it's not necessary

https://github.com/llvm/llvm-project/pull/90587